A knee joint prosthesis typically comprises a femoral component and a tibial component. The femoral component and the tibial component are designed to be surgically attached to the distal end of the femur and the proximal end of the tibia, respectively. The femoral component is further designed to cooperate with the tibial component in simulating the articulating motion of an anatomical knee joint.
Motion of a natural knee is kinematically complex. During a relatively broad range of flexion and extension, the articular or bearing surfaces of a natural knee experience axial rotation, adduction and abduction, translation in the sagittal plane (rollback and sliding), and translation in the coronal plane. Knee joint prostheses, in combination with ligaments and muscles, attempt to duplicate this natural knee motion, as well as absorb and control forces generated during the range of flexion.
One type of knee joint prosthesis that is commonly known is a fixed bearing knee. In a fixed bearing knee, a femoral component is attached to a resected portion of the femur and a tibial component is attached to a resected portion of the tibia. The fixed bearing knee also includes a bearing member having a integral stem that is fixed to the tibial component. The bearing member allows articulation of the femoral component along a bearing surface formed on the bearing member. As the joint moves into flexion, the bearing surfaces and the post cause the femoral component to rollback and slide in the sagittal plane. The movement in the sagittal plane increases the mechanical advantage, and thus the efficiency of the quadriceps. However, fixed bearing knees sometimes exhibit drawbacks. First, increased contact stresses may be created between the femoral component and the bearing member. The stresses are caused by relatively limited conformance of the femoral component to the bearing surface during flexion of the joint. Second, fixed bearing knees may become dislocated if the bearing member “jumps” off of the post. Lastly, the post may reduce the length of the trochlea groove which may lead to clunking of the patella.
Another type of knee joint prosthesis that has proven effective especially with respect to rollback is a floating bearing knee. In a floating bearing knee, a femoral component is attached to a resected portion of the femur and a tibial component is attached to a resected portion of the tibia with a floating bearing positioned therebetween. The floating bearing includes bearing surfaces that articulate with both the tibial component and the femoral component. As the joint moves into flexion, the ligaments and soft tissues of the knee, as well as a fixed post help to translate the floating bearing posteriorly causing rollback of the femur and leading to proper positioning of the patella, increasing the extension moment arm and increasing quadricep efficiency. However, floating bearing knees may also experience various drawbacks. Just like fixed bearing knees, floating bearing knees may experience dislocations of the femoral component to the tibial component and clunking of the patella during flexion of the joint.
Knee joint replacements are also sometimes unable to utilize all of the natural ligaments and the soft tissue supporting the knee joint. Depending on the degree of damage or deterioration of the ligaments, it may be necessary for a knee joint to eliminate one or more of motions of the knee joint in order to provide adequate stability. The use of a mechanical replacement ligament with a knee joint prosthesis is therefore desirable.
What is needed then is a knee joint prosthesis that has a mechanical ligament. This in turn, will provide femoral rollback relative to the tibial component by means of the mechanical ligament, provide a mechanically reconstructed ligament to constrain undesired translation of the knee joint, increase the overall reliability of the knee joint prosthesis, and prevent anterior movement of the bearing. It is, therefore, an object of the present invention to provide a fixed or floating bearing knee joint prosthesis having a mechanically reconstructed ligament that achieves the above-identified advantages.